Resistive volatile/non-volatile floating electrode logic/memory cell

ABSTRACT

A resistive floating electrode device (RFED) provides a logic cell or non-volatile storage or dynamic or static random access memory on an extremely compact matrix with individual cells scalable to the minimum available lithographic feature size regime by providing atomic switches connected in anti-parallel relationship, preferably with a common inert electrode. Programming is facilitated by limiting current to a compliance current level in order to maintain an OB state from which the cell can be written to either the 0 or 1 state. A perfecting feature of the invention provides for selective operation of a cell as a diode or in a volatile or non-volatile storage mode within the same memory array. A series connection of three or more RFEDs in accordance with the invention having different ON state currents, OFF state currents and reset currents can be used as adaptive, neural or chaotic logic cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority of U.S. ProvisionalApplication 61/510,736, filed Jul. 22, 2011, U.S. Provisional PatentApplication 61/539,575, filed Sep. 27, 2011, and U.S. Provisional PatentApplication 61/671,875, filed Jul. 16, 2012, and InternationalApplication PCT/US12/47846 filed Jul. 23, 2012, all of which are herebyincorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention generally relates to electronic logic circuits andmemory cells and, more particularly, to resistive, inert electrodedevices which can be fabricated in densely integrated arrays.

BACKGROUND OF THE INVENTION

In semiconductor integrated circuits, smaller minimum feature sizeregimes (generally limited by lithographic resolution) and greaterintegration densities have been continually sought in order to reducesignal propagation time and increase switching clock rates as well as toimprove noise immunity. High integration density also allows morefunctionality to be provided on a chip of given dimensions as well assubstantial economy in manufacture to develop a given level offunctionality. However, semiconductor integrated circuits based ontransistor switches are approaching the theoretical limits on minimumfeature size and maximum integration density. Further, increases inintegration density and switching clock rates are requiring operation atvery low voltages and currents in order to reduce heat dissipationrequirements for logic circuits and so-called support circuits instorage devices operated at high clock rates. Even when the vastmajority of transistors on a chip are formed at minimum feature size andoperated at low voltages and currents, a substantial number oftransistors on the chip must be formed at larger sizes and operated athigher voltages and currents for particular purposes such asinput/output (I/O) drivers, on-chip voltage regulators, drivers forbusses and large fan-out logic circuits and the like.

A particular problem is presented by the fact that many integratedcircuit logic devices such as microprocessors require some storage whichcan be accessed in a very few extremely short clock cycles and thus thestorage must be supplied on the chip. At the current state of the art,such storage for changeable data is provided as dynamic random accessmemory (DRAM) or static random access memory (SRAM) which are verydifferent structures with very different properties. DRAM cellsgenerally comprise only a small capacitor structure and a singletransistor which can generally be formed substantially above thecapacitor. Therefore, the memory cells can be very small and integrationdensity is generally governed by the spacing between the capacitors thatis needed for adequate isolation. However, such memory structuresrequire refreshing at frequent intervals since the amount of changestored on a given capacitor structure is very small and the transistorsas well as the capacitor structures are subject to leakage. Refreshoperations can occupy a significant portion of the operation time of theDRAM and can limit access time. Sensing of the stored charge alsorequires a significant amount of time since such sensing is generallyperformed by using the stored charge (or lack of stored charge) tounbalance a bistable circuit which has been balanced between stablestates and, after a read operation, the memory cell state must berewritten. Therefore, response time of a DRAM is relatively slow.

Where memory response time is critical and must be performed rapidly,SRAMs are generally employed. Instead of storing data as charge on acapacitor structure as in DRAM cells, an SRAM cell is formed as abistable transistor circuit, generally by cross-coupling the outputs andinputs of a pair of inverter circuits and including an additional pairof transistors for memory cell selection. Therefore such SRAM cells canbe formed using four (with two additional passive resistors in theinverter circuits), six or eight transistors or more which infers asignificant increase in chip area occupied by an SRAM cell.Additionally, in practice, the wiring to provide the cross-coupling ofthe inverters and the preferred orientation of the transistors (to havethe conduction paths in the same direction for more uniform conductioncharacteristics) requires substantial further chip space. A layout for asix transistor SRAM cell that is considered optimally compact thusrequires ten top twelve time the area required for a single transistor,which, as alluded to above, appears to be reaching the practical limitsof size reduction.

There has also been continuing interest in non-volatile memories whichare devices that can retain stored information substantiallyindefinitely without power being applied. This capability providessubstantial advantage in terms of convenience and/or security sincedata, once written, is permanently stored on a device independently ofany power supply which may be lacking or subject to power interruptionsuch as discharge or changing of batteries in portable devices.So-called floppy disks were an early expedient for providing such afunction but were limited in storage capacity while being somewhatlarger than might be convenient and subject to damage. Optical disksprovided greater storage capacity but were also relatively large and,like floppy disks, required a complex and expensive device for readingstored data while having the disadvantage of not providing for data tobe changed.

More recently semiconductor-based non-volatile memories have beendeveloped that are much smaller and can be read electronically withoutrequiring a reading device. Such devices have been used widely inelectronic devices such as digital cameras, cellular telephones andmusic players as well as in general computer systems, embedded systemsand other devices that require persistent storage. Such devices oftentake the form of removable and portable memory cards and storagecapacities of tens of gigabytes are available at low cost. However,semiconductor-based non-volatile memories such as flash memories andelectrically erasable programmable read only memories (EEPROMs) requiresemiconductor structures and operations which are of limited scalabilityand integration density and not optimal in speed and are thus notwell-suited to some applications.

Therefore, other types of structures are being investigated as potentialalternatives to transistors for memory cells and some logic circuits, inparticular, devices that can store data as differing resistance. Amongthese devices is a so-called phase change RAM (PCRAM) using achalcogenide element as a variable resistor and a resistance RAM (ReRAM)that uses a transition metal oxide element. One structure that has beenproposed in the last few years is a conductive bridge RAM (CBRAM) basedon a capacitor-like structure also developed in recent years andsometimes referred to as a memristive switch or an ionic or atomicswitch which changes resistance by precipitating metal cations to form aconductive bridge and ionizing the precipitated metal to destruct thebridge. The capacitor-like structure comprises two opposed plates ofdiffering materials (e.g. metals) with a dielectric material betweenthem that also exhibits electrolytic properties. One of the opposedplates is of an oxidizable material or metal (referred to as active)such as copper or silver and the other of a substantially inertconductive material or metal such as platinum or tungsten. A suitabledielectric material having electrolytic properties is tantalum oxidesuch as Ta₂O₅ or an oxygen deficient form thereof denoted by TaO_(x).Such a structure is initially non-conductive. However, when a suitablebias voltage is applied to the respective opposed plates, ions of theactive metal (e.g. Cu⁺) are extracted from the active metal plate anddrift-diffuse through the electrolyte and are stopped by and accumulateon the inert electrode. As ion drift-diffusion continues, active metalbuilds up on the inert electrode forming filaments (sometimes referredto as nanofilaments) that eventually reach the active metal electrodeand the device abruptly becomes conductive. This process is reversible,causing the nanofilaments to rupture, returning some of the active metalions to the active metal electrode and returning the switch to anon-conductive state, and, depending on the active metal andelectrolyte, exhibits sharply defined voltage thresholds at which theconductive filament formation occurs.

As alluded to above, a structure comprising two such atomic switchesformed back-to-back such that only a single inert electrode which can beallowed to electrically float is provided in common for both atomicswitches has been proposed as a non-volatile memory (NVM) cell or logiccircuit that does not require transistors as part of the storagestructure. This structure is referred to as a resistive, floating inertelectrode device or RFED. The potential for miniaturization is clearsince it is only required that some finite but arbitrarily small area beprovided and the two atomic switches can preferably be formed in avertical orientation (the respective atomic switches being then referredto as upper and lower switches; a convention that will be usedhereinafter for convenience but without any inference of relativeorientation of the atomic switches being intended) and theoreticallyprovide four electrical states (e.g. two different resistive states foreach atomic switch) corresponding to two bits of information.

However, three of these states (e.g. where no nanofilaments are formedin either atomic switch or nanofilaments are formed in only one of theatomic switches) are difficult to distinguish in a two terminal devicesince all three states are of high impedance and the switch in whichnanofilaments are formed (or ruptured) cannot be discriminated. Further,as proposed, such RFEDs cannot be formed as arrays where connection ofRFEDs may cause so-called sneak circuits to other RFEDs through theinert electrode or dielectric electrolyte. Such sneak circuits can alsobe caused in an array of RFEDs fabricated in an optimally compactcross-bar configuration by the easily distinguishable fully conductivestate where nanofilaments exist in both atomic switches of an RFED.Additionally, relatively strong electric fields are required to causesufficient electromigration or drift-diffusion to form or rupturenanofilaments. Also, as a practical matter, volatility/persistence ofstorage is relatively unstable; tending to vary with the past writinghistory of the individual RFED cell. By the same token writing operationto an RFED cell may require extended highly variable periods of time;again depending on the previous storage state of the RFED cell. The RFEDcell also tends to generate pulses if the set and reset thresholdspresent only a small operating window. In conventional circuitry, alarge number of transistors is needed to generate a current or voltagepulse. With an RFED, only one highly scalable device is needed. Due toat least these problems which have been largely intractable, RFEDs havenot been widely studied or developed. Therefore, at the current state ofthe art, RFEDs do not provide a practical alternative to transistors inlogic circuits and storage devices.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an RFEDstructure that can be fabricated in a compact array and operated as apractical storage device that does not require transistors and can befabricated at an arbitrarily small minimum feature size in optimallycompact arrays.

It is another object of the invention to provide RFED cells that can beoperated as either volatile memory (SRAM or DRAM) cells or asnon-volatile memory cells in the manner of flash memory or floating gatetransistor devices.

It is another further object of the invention to provide logic circuitscomprising RFEDs that can be formed at small size in a compact arraythat achieve accelerated learning, and partial or complete unlearning asadaptive circuits that are highly simplified in configuration andconnection and occupy only a relatively few RFED sites in a compactcross-bar array of RFEDs.

In order to accomplish these and other objects of the invention, amethod of operating an atomic switch is provided comprising steps ofapplying a threshold voltage across the atomic switch to render saidatomic switch conductive, and limiting current through the atomic switchto a level less than a current required to render the atomic switchnon-conductive.

In accordance with another aspect of the invention, an atomic switch isprovided comprising an inert electrode, a δ-copper layer on said inertelectrode, an active or inert electrode spaced from the δ-copper later,and a solid dielectric/electrolyte filling a space between the δ-copperlayer and the active or inert electrode.

In accordance with a further aspect of the invention, a logic device isprovided comprising three or more serially connected atomic switches,wherein the three atomic switches exhibit OFF resistances, ONresistances and reset currents that differ from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a schematic cross-sectional view of a device referred to as anatomic, ionic or memristive switch,

FIGS. 2A and 2B illustrate the operation of the atomic switch of FIG. 1,

FIGS. 3A and 3B illustrate the current/voltage (I-V) characteristics ofthe atomic switch of Figure as the switch made conductive andnon-conductive, respectively,

FIGS. 4A and 4B illustrate alteration of the hysteresis characteristicsof the atomic switch by use of different electrolyte and/or electrodematerials,

FIGS. 5A and 5B illustrate the structure of a resistive floatingelectrode device (RFED) from two atomic switches,

FIGS. 6, 7A and 7B illustrate programming of the RFED of FIG. 5A or 5B,

FIG. 8A is a cross-sectional view of an RFED cell as fabricated in amatrix array,

FIG. 8B is a plan view of a portion of a cross-bar matrix array ofRFEDs,

FIGS. 9A, 9B and 9C show experimentally derived electricalcharacteristics of the RFED of FIG. 8A,

FIGS. 10A and 10B schematically illustrate cross-sections of variantembodiments of an RFED,

FIG. 11 illustrates operation of an RFED in accordance with theinvention.

FIG. 12 illustrates possible states of an RFED,

FIG. 13 illustrate an I-V plot of an RFED with a small operating rangeor window,

FIG. 14 illustrates application of a compliance current to an RFED,

FIGS. 15 and 16 illustrate operation of an RFED with application of acompliance current,

FIGS. 17 and 18 illustrate operation of an RFED using a compliancecurrent,

FIGS. 19 and 20 illustrate improvement of operation of an RFED bydecoupling set and reset thresholds using a compliance current,

FIGS. 21 and 22 illustrate writing and reading operations of an RFED fordiscriminating different non-conductive states,

FIGS. 23, 24, 25 and 26 illustrate an atomic switch that exhibitscontrollable volatility and the operation thereof,

FIGS. 27, 28, 29 and 30 illustrate electrical characteristics of acontrollable volatility atomic switch.

FIG. 31 schematically illustrates a variant form of a controllablevolatility atomic switch,

FIGS. 32, 33A, 33B and 33C illustrate an exemplary logic array of atomicswitches that provide for controlled learning and unlearningprogrammable states,

FIG. 34 illustrates exemplary waveforms for selectively programming thecircuit of FIG. 32, and

FIGS. 35A and 35B illustrate a logic circuit similar to FIG. 32providing accelerated learning and forced and selective unlearning.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown an exemplary structure of a so-called atomic, ionic or memristiveswitch. Since this switch is of exemplary materials and other structuresmay be made to operate in a similar fashion, and the exemplary materialsillustrated have been chosen to facilitate an understanding of theinvention and relationship of preferred features thereof no portion ofFIG. 1 (or FIG. 2A, 2B, 3A or 3B is admitted to be prior art in regardto the invention and these illustrations have, accordingly, beendesignated as “Related Art”.

The atomic switch, as illustrated in FIG. 1 comprises a capacitor-likestructure with an active metal plate 10 formed of, for example, copper,a dielectric material 20 that also exhibits electrolytic properties suchas oxygen deficient tantalum oxide (TaO_(x)) and an inert metal plate 30formed of, for example, platinum. Electrical connections are made to theopposed active metal and inert metal plates as schematicallyillustrated. However, the details of electrical connections are of noimportance to an understanding of the invention in accordance with itsbasic principles but only in regard to forming a compact array of RFEDsas will be discussed in greater detail below.

When a suitable bias voltage is applied to the opposed plates, ions 40of the active metal are extracted from the active metal plate anddrift-diffuse through the dielectric/electrolyte 20 as depicted by arrow45 in FIG. 2A. These ions are stopped by and collect on the inertelectrode 30 and form a deposit 50. As the process continues, theelectric field strength in dielectric/electrolyte 20 will be increasednear conductive clustered copper deposit 50 and further preferential iondeposition thereon will be enhanced; causing the formation ofnanofilament 55 as shown in FIG. 2B which eventually forms a conductivebridge between the two opposed plates and results in a low impedancepath between the opposed plates. It should be noted, for a oxygendeficient dielectric/electrolyte such as TaOx (but not others, such asstochastic materials, that are not oxygen deficient) as illustrated,there will be a similar drift-diffusion and collection of doublynegative charged oxygen vacancies from the electrolyte toward the activemetal electrode that can similarly form a conductive bridge between theopposed plates. Even so, the charge transfer is extremely small and theconductivity remains substantially zero until a conductive bridge isformed. The mechanism of oxygen depletion drift-diffusion may or may notbe present, depending on the choice of dielectric/electrolyte and isonly mentioned here for reference in the discussion of variantembodiments of the invention as will be provided below.

The alteration of electrical properties of the atomic switch areillustrated in FIGS. 3A and 3B. FIGS. 3A and 3B are graphical plots ofvoltage applied across the atomic switch and the resulting currentthrough the atomic switch (referred to as I-V characteristics) withschematic depiction of the formation and rupturing of a nanofilamentbridge to aid in visualizing the operation of the atomic switch toresult in the depicted I-V characteristics illustrated. Initially,before any voltage is applied and before any active metal extraction ordrift-diffusion occurs, the atomic switch is non-conductive for anysmall bias voltage that may be applied to the opposed plates 10, 30, asdepicted by the horizontal body of arrow 60 in FIG. 3A. If the voltageis swept to a negative voltage, the current will remain substantiallyzero until a critical voltage V_(th-on) is reached; at which voltageactive metal ions will be extracted from plate 10, drift-diffuse throughthe dielectric/electrolyte 20 and collect on inert electrode 30 to forma conductive nanofilament that form a conductive bridge between theopposed plates. At this point, the atomic switch becomes abruptlyconductive as depicted by arrow 65 in FIG. 3A. For greater bias voltagesafter this point, the electrical behavior of the atomic switch will beohmic as indicated for larger negative bias voltages by arrow 70 of FIG.3A and for smaller negative and small positive bias voltages by arrow 75in FIG. 3B. However, as the bias voltage becomes positive, thedrift-diffusion of active metal ions toward the inert electrode ceases(and may, in some cases, spontaneously reverse) and as positive biasvoltage is increased, active metal ions may tend to drift-diffuse in theopposite direction, returning to active electrode 10, and at somepositive voltage bias, V_(th-off), the nanofilaments will rupture;causing the atomic switch to become non-conductive once again asdepicted by arrow 80 in FIG. 3B although some drift-diffusion maycontinue for positive bias until a significant portion of the activemetal ions are returned to active electrode 10.

The bias voltage excursions for performance of a complete switchingcycle of an atomic switch as shown in FIGS. 3A and 3B are depicted inFIG. 4A. However, ohmic behavior for larger negative bias voltages 70required for turn-on as illustrated in FIG. 3A are omitted in FIG. 4A,for clarity. Arrows 65 and 80 at V_(th-on) and V_(th-off), respectively,are the two characteristics of the hysteresis exhibited by the atomicswitch. It is important to observe that the voltages at which thesethresholds occur are determined by the material properties of the activeelectrode metal (such as work function and the parameters of the redoxreaction at the type of the active metal and electrolyte interface) anddielectric/electrolyte and its thickness. That is, if a differentdielectric/electrolyte and suitable active metal electrode are chosen,the characteristic voltages of the hysteresis will be qualitatively thesame (e.g. producing a “bow-tie” array of arrows 65-80) but therespective threshold voltages will have different values. Therefore, itis desirable, in RFED devices as will be discussed below, to shift thecharacteristic threshold voltages to the right (e.g. to combine thenegative voltage hysteresis with part of the positive voltage hysteresisas shown in FIG. 4B, particularly when two atomic switches are combinedto form a RFED in order to be able to form bridges in both the upper andlower atomic switches of the RFED and to have the bridges existconcurrently and form and rupture in an advantageously controllablemanner). It should be appreciated in this regard that when two atomicswitches are connected in electrically opposing relationship (e.g. instructurally opposite orientations), referred to as anti-serial, orformed with a common inert electrode, shifting of the threshold voltagesto the right, as shown, for either or both of the atomic switcheseffectively expands the voltage range of ohmic behavior when bridgeshave been formed in both atomic switches, increases the write and/orerase threshold for improved noise immunity, particularly when thedevice is used as a memory device and may reduce the likelihood ofspontaneous rupturing of the nanofilaments when the bias voltage ishigh. Such a threshold shift can also be altered by electrode spacingeither alone or in combination with choice of dielectric/electrolyteand/or electrode materials but increases in electrode spacing willreduce the speed with which the development of a nanofilament bridgebetween the electrodes can be accomplished.

Referring now to FIG. 5A, an exemplary preferred structure of aresistive floating electrode device (RFED) 90 in accordance with theinvention will now be discussed. FIG. 5A schematically illustrates twoatomic switches of similar construction but which include differentexemplary dielectric/electrolyte materials. Specifically, in thisexample, atomic switch A, 91, is identical to the exemplary atomicswitch discussed above in connection with the illustrations of FIGS.1-4A. Atomic switch B, 92, is of similar construction but includesstochastic tantalum oxide, Ta₂O₅, rather than oxygen deficient tantalumoxide, TaO_(x), as the dielectric/electrolyte. Accordingly, atomicswitch B, 92, will exhibit a shift in voltage thresholds as comparedwith atomic switch A, 91, for forming and rupturing the nanofilamentbridges as discussed above in regard to FIG. 4B. Atomic switch B, 92, isalso illustrated in an inverted orientation relative to atomic switch A,91, with the inert electrode at the top and the active electrode at thebottom. This inversion of orientation also reverses the polarity of biasvoltage applied to atomic switch B relative to the voltage applied tothe overall RFED device. It should be noted that the connection betweensingle switches is provided by the merging of the two inert (Pt)electrodes into one common, floating inert electrode. However, the twoinert electrodes can be connected externally in any convenient way torealize an electrically equivalent RFED structure. These two atomicswitches can be connected as shown by dashed line 93 to Rain an RFED.One preferred form of such a connection is simply to provide a singleinert electrode that is common to both atomic switches, as shown in FIG.5B to form RFED 95. While the inert electrode 94 is illustrated ascomprising platinum throughout its volume, it should be understood thatelectrode could be an alloy or other material or even a layeredstructure of different substantially inert conductive materials forpurposes such as adjusting thresholds as discussed above in connectionwith FIG. 4B, or to develop other desired electrical or physicalproperties of the RFED.

Referring now to FIG. 6, operation of a RFED such as that depicted inFIG. 5B will now be discussed. As with FIGS. 3A and 3B, FIG. 6 is agraphical plot of voltage applied to the RFED and the resulting currenttherethrough with inset schematic depictions of the RFED andnanofilament bridge formation and rupture. As in FIG. 6, before any biasvoltage is applied to the RFED, no nanofilament bridges will have beenformed and the state of the RFED will be as depicted by circle 1. As apositive bias is applied, at a threshold 96, a first nanofilament bridgewill be formed in one of the atomic switches of the RFED. However, sincethe nanofilament bridge is formed in only one of the atomic switches asdepicted at circle 2, the RFED remains non-conductive and the currentsubstantially zero even at positive bias voltages substantially greaterthan the threshold voltage (but below a breakdown voltage of the entireRFED device or of the individual atomic switches). (Slight variationfrom zero current is depicted to allow the voltage excursion to be moreeasily followed.) If the bias voltage is then swept to a negativevoltage, when a negative threshold 97 is reached, a nanofilament bridgeis formed in the other atomic switch as depicted at circle 3 and theRFED becomes conductive. (The second bridge is formed at a bias polarityopposite to the threshold causing formation of the first bridge sincethe atomic switches are connected anti-serially.) The first monofilamentbridge formed at threshold 96 remains in place due to the adjustment(e.g. increase or shift to the right) of thresholds as discussed abovein connection with FIG. 4B. If the bias voltage is swept to even highernegative voltages, threshold 98, referred to as the switch A resetvoltage, V_(th-reset(A)), is reached and the first nanofilament bridgeis ruptured as shown at circle 4 and the RFED again becomesnon-conductive. (The difference in thresholds between circle 3 andcircle 4 and the opposite effect achieved at the respective thresholdsis due to the combination of threshold adjustment of one or both atomicswitches as well as the anti-serial connection alluded to above.)Between the states of the RFED developed at circle 3 and circle 4 whilethe RFED is conductive, the RFED will exhibit ohmic characteristics asshown at 99 on the I-V plot. If the bias voltage is again swept to apositive voltage, the second monofilament bridge will be ruptured at athreshold 100 which is less than threshold 96, as shown at circle 5,again because of adjustment of thresholds of the respective atomicswitches as discussed above.

Alternatively, as shown in FIG. 7A, if threshold 98 is not reached afterthe RFED is rendered conductive at the state of circle 3 of FIG. 6, theRFED will exhibit ohmic behavior even at positive voltages belowthreshold 100 (e.g. for voltage excursions between the voltagesindicated by circle 3 of FIG. 6 and circle 6 of FIG. 7A since the stateof the RFED is the same as that achieved at circle 3 of FIG. 6. However,at a bias voltage threshold 100, referred to as a switch B resetvoltage, V_(th-reset(B)), the second nanofilament bridge to be formed atstate circle 3 of FIG. 6 is ruptured while the first Nanofilament bridgeto be formed at the state of circle 2 of FIG. 6 remains in place, asshown at circle 7. Conversely, as shown in FIG. 7B, between statesindicated by circle 6 and circle 8, ohmic behavior will be exhibited atpositive or negative bias voltages below V_(th-reset-B) and above (e.g.less negative) than V_(th-reset(A)) while at that threshold, the firstnanofilament bridge to be formed is ruptured as depicted at circle 9.Therefore, it is seen that once the RFED is rendered conductive, eithernanofilament bridge can be selectively ruptured.

It will be recalled from the foregoing that, while four storage statesof an RFED are theoretically possible and could theoretically providethe storage equivalent of two bits in a single RFED, the conductivestate cannot be used when plural RFED devices are connected in andaccessed through a matrix type of connection arrangement since theconductive state of an RFED will cause a sneak connection paths to otherRFEDs in the other rows and columns of the matrix. It will also berecalled that the three non-conductive states of an RFED cannot bereliably distinguished and cannot be discriminated in any practical way.Moreover, without shifting of threshold of one or both atomic switchesof an RFED in accordance with the RFED of the invention, the developmentof nanofilament bridges in both of the atomic switches cannot bereliably achieved and thus there is no selectivity, much lessreliability, in forming a single nanofilament bridge in only one of theatomic switches to allow the two “single bridge” states of an RFED to beused as storage states or, even if selectively achieved, to bediscriminated. The RFED in accordance with the invention not only allowsthe two “single bridge” states to be reliably and selectively achieved,as discussed above, but exploits the conductive state to enhance thewriting operation to the RFED and discriminating between the two “singlebridge” states as will be discussed below, while limiting the durationof the conductive state to periods where possible sneak circuit pathsare of no effect since all unselected RFED cells are in one of the“single bridge” states and, hence, non-conductive, allowing fabricationof RFEDs in highly compact matrix arrays; an exemplary preferred form ofwhich will now be discussed in connection with FIGS. 8A and 8B.

FIG. 8A illustrates a cross-section of a RFED cell construction suitablefor matrix fabrication. In this example, the RFED is inverted ascompared with FIG. 5B. The orientation of the RFED in accordance withthe invention is not important to the understanding and practice of theinvention in accordance with its basic principles but can provideflexibility and convenience in design for some applications where morethan two atomic switches are provided in a stack for artificialintelligence and adaptive circuit applications and the like as will alsobe discussed in greater detail below in regard to several perfectingfeatures of the invention.

Specifically, an insulative substrate (or layer on a substrate or someother structure) such as of thermally oxidized silicon wafer is providedon which a bottom active electrode 120 such as copper is deposited andpatterned into elongated conductors extending in the direction of theplane of the page and terminating in connection pads 170 as shown in theplan view of a compact cross-bar matrix provide in FIG. 8B. A firstdielectric/electrolyte layer 130 is then deposited on the activeelectrode 120. The thickness of the dielectric/electrolyte layer ispreferably kept small (e.g. on the order of a few tens of nanometers) toallow conductive bridges to be rapidly formed and ruptured. Nopatterning of layer 130 is required but could be provided if desired.Inert electrode 140 is then deposited as a blanket layer and thenpatterned into discrete (e.g. rectangular) areas so that contact withany other electrically conductive structure can be avoided. These areascan be made as small as is reasonably feasible for acceptablemanufacturing yield. At the present state of the art, a transversedimension of these areas can be as small as a few nanometers and smallerdimensions are foreseeable. Insulating material can be deposited orgrown in the space between the bodies of inert electrode material forisolation, if desired by known and well-understood processes inaccordance with the chosen insulating material. The seconddielectric/electrolyte material 150 is then deposited over the bodies ofinert electrode material. Again, this dielectric/electrolyte layer iskept to substantially the same dimensions as the firstdielectric/electrolyte layer 130 (but either thickness could be variedto trim capacitance) and patterning is not necessary but could beperformed, if desired. Finally, another active electrode material layeris deposited and patterned to form elongated conductors extending, forexample, perpendicular to the plane of the page and terminating inconnection pads 180 as shown in FIG. 8B. The method of deposition of theelectrode and dielectric/electrolyte layers is not at all critical tothe practice of the invention and many known and foreseeable techniquesare considered suitable. However, sputtering or electron beamevaporation is currently considered somewhat preferable for goodthickness control of the dielectric/electrolyte layers.

FIGS. 9A, 9B and 9C show experimental results of the electrical behaviorof an exemplary RFED constructed as shown in FIGS. 8A and 8B. FIG. 9Ashows a sweep from zero volts to V_(th-set-A) (at about 3V) during whichthe first bridge is created as discussed above in connection with FIG.6, followed by sweeping the voltage to a negative voltage. At a negativethreshold of about −4.3 V an abrupt increase in negative current isobserved as the second bridge is formed at V_(th-set-B) and the RFED isrendered conductive. As shown in FIG. 9B, ohmic behavior is observed atsmall bias voltages between the two thresholds. This ohmic behavior atsmall bias voltages is also shown in FIG. 9C for voltages between −0.9Vand 1V. However, as the voltage is swept to −2V, at a bias voltage ofabout −0.9V the current abruptly drops to zero, indicating the ruptureof the nanofilament bridge in atomic switch A. Similar behavior isobserved for larger positive bias voltages; indicating good experimentalagreement with the RFED structure and programming operation as discussedabove.

The difference in voltage between the initial formation of nanofilamentbridges, referred to as a forming voltage, and the rupturing thereof aswell as a reduced voltage for reformation of nanofilament bridges is dueto the fact that a higher voltage is required to initially form theentire bridge while some portions of the bridge will already exist whenthe bridge is re-formed and the fact that once one nanofilament bridgeis in place, the inert electrode is no longer floating, whereas beforeany nanofilament bridge is formed, the RFED will constitute a capacitivevoltage divider and the floating inert electrode will assume a voltagenear one-half of the bias voltage applied to the RFED, depending of thedielectric constant of the dielectric/electrolyte(s) and the electrodespacing. When both nanofilament bridges are in place, the RFED willeffectively be a resistive voltage divider.

Referring now to FIG. 10A, a variant form of atomic switch that can beemployed in a RFED in accordance with the invention is shown. In thiscase, both electrodes are of an inert material such as platinum. Thedielectric/electrolyte can be any of a wide variety of insulative oxidessuch as iron oxide (FeO). When a bias is applied, doubly charged oxygenvacancies, illustrated by open circles, are created and drift-diffuse inthe same manner (but opposite direction) as described above for activemetal ions to form connecting, conductive bridge(s). This variation ofatomic switch structure effectively mixes nanofilament conductive pathswith metal ions and oxygen vacancies such that resistive switching takesplace at the entire interface between the oxide and the electrode. Anexemplary RFED incorporating such an atomic switch is shown in FIG. 10Busing a combination of the atomic switch of FIG. 1 and the atomic switchof FIG. 10A. These illustrations should be considered as exemplary ofother variations of the RFED in accordance with the invention whichcould include atomic switches with the same dielectric/electrolytematerial but employing different electrode materials or electrodespacing, use of other dielectric/electrolyte materials such as aluminumoxide, using a layered structure of different materials (e.g.Ta₂O₅/TaSiOx/Al₂O₃) for the dielectric/electrolyte, providing one atomicswitch as a unipolar mode switch in which rupturing of the conductivefilament is due to Joules heating that is independent of currentdirection or any combination of such variations.

As briefly discussed above, in known RFEDs (e.g. without threshold shiftof one or both atomic switches) the fully conductive state with twoconductive bridges in place could not be reliably achieved or maintainedsince the bias voltage polarity (and slight current) required to formthe second bridge, as discussed above, causes active metal (and/oroxygen vacancies) to be removed from the first bridge, leaving only avery small and critical voltage window, or no voltage window at all, forformation of the second bridge without rupture of the first bridge. Inother words, in an RFED, the set threshold for one atomic switch may bevery close to or the same as the reset threshold for the other atomicswitch. In any event, as noted above, the conductive state or an RFEDcould not be used as a memory state when a plurality of RFEDs arefabricated in a matrix array and that the three possible non-conductivestates could not be discriminated in any way that is practical for ahigh capacity memory device formed at high integration density andproviding write and read times comparable to transistor-based memorystructures. The reliable and selective formation of conductive bridgesin both atomic switches in an RFED however, can be achieved byadjustment of threshold voltages of the two atomic switches, asdiscussed above. However, this capability does not solve the problem ofdiscriminating which of the two atomic switches has a conductive bridgein place.

This problem is solved in accordance with the invention by the use of acompliance current which not only allows discrimination of which of thetwo atomic switches has a conductive bridge in place while the RFEDremains in a non-conductive state so that the state of the RFED, whenused as a memory device or in some logic applications, can be read butalso allows selective formation of a conductive bridge in either atomicswitch of an RFED and for that state to be read in an RFED. Both forsimplicity of description and to demonstrate the robustness of thecompliance current technique, the following description will be madewith reference to an RFED in which the set and reset thresholds of bothatomic switches are identical. However, the compliance current switchingtechnique is identically applicable to RFEDs where the respectivethresholds of the atomic switches are adjusted to be different.Additionally, the compliance current technique can provide decoupling ofset and reset thresholds even when very similar and thus a small ornon-existent operation window between the threshold voltages is avoided.Further, the compliance current technique can be advantageouslyexploited to achieve additional electrical effects in connection with afurther perfecting feature and embodiment of the invention as will beexplained below.

Essentially, the compliance current technique is a limitation on thecurrent allowed to pass through the RFED when in the ON or conductivestate to a level below the reset current and provides for maintenance ofthe ON state indefinitely while allowing for a variable voltage dropacross the device. The ON state can then be changed to the 1 or 0 stateby exceeding a threshold voltage for bridge formation but not exceedingthe current above the reset current for either atomic switch.

To convey an understanding of the compliance current technique ofoperating a RFED, reference is first made to FIG. 11 illustrating anexemplary hysteresis characteristic of an atomic switch. Thisillustration essentially combines FIGS. 3A and 3B as discussed above. Inthe following discussion, V_(th-on) and V_(th-off) are used for a singleswitch whereas V_(set) and V_(reset) are used to refer to the entireRFED. V_(set) is approximately V_(th-on). However, V_(reset) is about2V_(th-off).

From an initial non-conductive state at zero volts, the voltage can beswept through increasingly negative voltages until a set threshold,V_(set), is reached; at which point a conductive bridge will be formedand the atomic switch will become conductive and exhibit ohmic behaviorfor all negative bias voltages and small positive bias voltages below areset threshold, V_(reset). At V_(th-off), the conductive bridge will beruptured and the atomic switch will become non-conductive for allvoltages above V_(th-on). It should be noted that the hysteresischaracteristic is slightly asymmetrical, as is typical of atomicswitches of various constructions and the set and reset currents as wellas the set and reset voltage thresholds will differ accordingly. In thisregard, it should be noted that the set voltage that is applied must besmaller in magnitude than the reset voltage since the RFED could not beoperated as a storage device if that were not the case since an ON statecould not be achieved and the reset current achieved if the firstconductive bridge is ruptured before the second conductive bridge isformed. The extent to which the reset threshold magnitude exceeds theset threshold magnitude is the operating window of the RFED.

It should also be noted in this regard, that if the necessary conditionof the magnitude of the reset voltage being greater than the magnitudeof the set threshold, when one conductive bridge exists the voltages atwhich the second bridge will be formed is above the set threshold due tothe voltage drop across the existing conductive bridge when the currentis increased as the conducting bridge is formed. This increase in theset voltage reduces the size (in volts) of the operating window of theRFED even though the magnitude of the reset voltage may approach twicethe magnitude of the set voltage. This effect is also illustrated inFIG. 15.

Referring now to FIG. 12, the four potential states of a RFED areillustrated. The initial state in which both the upper and lower atomicswitches have a high resistance state (HRS/HRS) is referred to as theOFF state which will not be resumed after a conductive bridge is formedin either the upper or lower atomic switches. The 0 state is defined asa conductive bridge and low resistance state (LRS) existing in only theupper but not the lower atomic switch and is denoted by the legend(LRS/HRS). Conversely the 1 state is defined at a state in which aconductive bridge and LRS exists only in the lower atomic switch but notthe upper atomic switch and is denoted by the legend HRS/LRS. Finallythe ON state is defined as a state in which a LRS and conductive bridgeexists in both the upper and lower atomic switch such that ohmicbehavior will be exhibited. The I-V plot for the 0, ON and 1 states ofthe RFED for either of the atomic switches without the application ofcompliance current is also illustrated in FIG. 12.

The I-V plot for the entire RFED during a voltage sweep is shown in FIG.13. From an initial zero (magnitude) voltage, the voltage is increaseduntil the set voltage (exceeding the set threshold as noted above) isreached the RFED becomes abruptly conductive and at only a slightlygreater voltage magnitude becomes abruptly non-conductive again over avoltage excursion of a small fraction of a volt. This behavior can befrequently observed in an experimental arrangement and is indicative ofthe criticality of the voltage operating window of an RFED which is dueto the fact that the set current and voltage drop across the firstformed conductive bridge is relatively large and the reset threshold forthe first formed conducting bridge is generally less than the setthreshold for either conductive bridge of an RFED. Therefore theoperating window is very small. Further, the large current in the ONstate allows rupture of the first-formed conductive bridge due toexcessive Joules heating. While this is disadvantageous for the RFEDprogram operation, it offers a beneficial capacity for generation ofshort current and voltage pulses.

Referring now to FIG. 14, application of a compliance current isillustrated. The I-V plot of FIG. 14 differs from the I-V plot of, forexample, FIG. 12 by limitation of positive or negative ON state currentto a level less than I_(reset). Such a limitation of ON state currentcan be achieved very simply by, for example, a resistance in series withthe RFED. This limitation of current at voltages near the set and resetthresholds adequately prevents the eroding or dissolution of conductivebridges that have been formed in the atomic switches of the RFEDregardless of the polarity or, within limits, the magnitude of the biasvoltage applied to the RFED which is thus maintained in the ON state. Itshould be noted that while the I-V plot of FIG. 14 illustrates asubstantial magnitude of the compliance current for clarity and thecompliance current can in fact, be a substantial fraction (e.g. one-halfto one-tenth) of the reset current but is, in theory, more effective andreliable when set to a value smaller than the reset current by a factorof ten to one hundred, for example 0.1 milliamperes.

FIGS. 15 and 16 illustrate a cycle of operation of an RFED withoutapplication of the compliance current and, for comparison, a cycle ofoperation of the RFED with application of a compliance current will beillustrated in FIGS. 17 and 18. Referring now to FIG. 15 and assumingthat the RFED is in a 0 (LRS/HRS) state with the upper conductive bridgein place, the bias voltage is swept from zero or a positive level to anegative level during which voltage excursion the current issubstantially zero. At a sufficiently negative voltage −V_(set) which issomewhat greater in magnitude −V_(th1) at which a conductive bridgewould be formed in an atomic switch (due to the voltage drop across theupper conductive bridge as the lower conductive bridge is formed), theRFED will assume the conductive ON state and as the bias voltage isswept to a more negative value, the current will increase ohmically. Ata more negative voltage −V_(th2), the upper conductive bridge isruptured and the RFED returns to a non-conductive 1 (HRS/LFS) state andwill remain non-conductive at even more negative bias voltages. FIG. 16shows substantially the reverse process superimposed on the I-V plot ofFIG. 15. As the voltage is swept toward positive voltages the RFEDremains non-conductive until a bias voltage of +V_(th1) is reached andthe upper conductive bridge re-formed causing the RFED or RFED to assumethe ON state in which current increases ohmically as bias voltage isfurther increased. when a bias voltage of +V_(th2) is reached, the lowerconductive bridge is ruptured and a non-conductive 0 (LRS/HFS) statewill be resumed.

Referring now to FIG. 17, operations on a RFED similar to thosedescribed above in connection with FIG. 15 are performed, starting withan assumed 0 (LRS/HRS) state but with compliance current limitation inthe ON state. Specifically, as the bias voltage is swept from a zero orpositive voltage toward a negative voltage, the RFED is non-conductivesince the lower conductive bridge has not been formed. At −V_(set), thelower conductive bridge is formed and the RFED assumes the conductive ON(LRS/LRS) state. However, since current is limited to the level of acompliance current, the ON state is maintained even at negative voltagesbeyond −V_(th2). At bias voltage levels near zero volts, the currentlevel may be allowed to vary ohmically between the positive and negativecompliance current levels while the ON state is, nevertheless,maintained.

The limitation of current to the compliance current level is notcritical but should be substantially below or a fraction of the resetcurrent, I_(reset), corresponding to the ohmic current level reached at±V_(th2) in the operations described in regard to FIGS. 15 and 16.Without wishing to be held to any particular theory of operation of thecompliance current to maintain the ON state under a broad range ofvoltage conditions, it appears that the limitation of current to acompliance current level during the ON state maintains the current levelsufficiently low that erosion of ions and/or oxygen vacancies from theconductive bridges and drift-diffusion back to the active electrode doesnot occur regardless of the magnitude of the bias voltage that isapplied. Thus, while an effective compliance current level can be asubstantial fraction of I_(reset), lower current levels should be evenmore effective and are highly desirable to limit ohmic heating and powerconsumption requirements. In fact, compliance current levels can bereduced to a small fraction of a milliampere. With current limited to acompliance current level, the ON state can be maintained indefinitelyand can be exploited to enhance both read and write operations as willbe discussed below. Since the compliance current level is not at allcritical, compliance current limitation can be achieved very simplyusing any of a number of constant current source circuits or devicesfamiliar to those skilled in the art, such as a MOSFET at saturation.

Referring now to FIG. 18, the RFED can now be written to either the 1 or0 state by removing the limitation of current if the magnitude of thebias voltage is greater than ±V_(th2) removal of the compliance currentlimitation will cause the RFED to immediately assume the 1 or 0 state,respectively. If the bias voltage is near zero when a write operation isdesired, the bias voltage is simply swept to ±V_(th1) to set the RFED toa 0 or 1 state, respectively.

It should be noted that the use of a compliance current can allow theset and reset voltages to be decoupled as shown in FIG. 19. Assume thatthere are only two initial states: 0 (LRS/HRS) and 1 (HRS/LRS). Eitherof these states can be changed to the ON state (LRS/LRS) by applying+V_(set)(>+V_(th1)) with a compliance current smaller than the singleatomic switch reset current. This ON state can then be changed to eithera 1 or 0 state at any voltage having a magnitude greater than ±V_(th2)if the compliance current limit is removed. Thus a potentially verynarrow operation window can be avoided and the RFED written to eitherstate at any arbitrary voltage having a magnitude greater than T_(th2).Further, the use of a compliance current limitation during the setoperation can prevent soft breakdown of the device and improve overalldevice reliability. That is, as shown in FIG. 20, the operating windowbetween the set and reset voltage magnitudes is diminished from thedifference between V_(th1) and V_(th2) due to the voltage drop in theexisting conductive bridge while the other conductive bridge is beingformed. Thus, if V_(th2) magnitude is not significantly different fromor less than V_(th1), the device cannot function properly as a memorydevice. Decoupling of the set and reset voltages allows writing atarbitrarily high voltage significantly different from the magnitude ofV_(th1), resulting in high reliability of the writing operation.

Referring now to FIG. 21, discrimination between the non-conductive 0and 1 states during a read operation will now be discussed. As shown inFIG. 21, the ON state may be entered at V_(set)>V_(th1) which can besupplied as a pulse, whether a limited compliance current is applied ornot. From the ON state, either the 1 or 0 state can be reached byapplication of a positive or negative pulse of suitable magnitude withthe compliance current limit removed. However, as shown in FIG. 22, apositive or negative pulse of magnitude V_(th2)>V_(set)>V_(th1) appliedto the RFED can serve as a read pulse to discriminate the 1 or 0 stateof the RFED. For example, if the RFED is in the 0 state and a positivepulse of magnitude V_(th2)>V_(set)>V_(th1) is applied, the ON state willnot be entered since the polarity is opposite the polarity which willcause a lower conductive bridge to be formed. The RFED or RFED willremain in the non-conductive 0 (LRS/HRS) state and only a very smallcurrent, if any, will be detected. If, however, the RFED is in the 1(HRS/LRS) state with the lower conductive bridge in place, a positiveread pulse would cause the upper conductive bridge to be formed, the ON(LRS/LRS) state will be entered and a significant current can bedetected to indicate a reading of the 1 state, in which case, theinformation is destroyed and the RFED must again be written to a 1 or 0state. However, such an operation can be performed in a simple mannersimilar to refresh of a DRAM cell but only after a reading operationsince storage is, for this structure, non-volatile. A similardiscrimination can be performed using a negative V_(th2)>V_(set)>V_(th1)pulse in which no current will be detected for a stored 1 state whichwill persist after the read operation but current will be detected uponentering the ON state from a 0 state.

It should be noted that if the RFED is in the intermediate, conductiveON state when a read pulse is applied the detected current will belimited by the compliance current level and the ON state can also bediscriminated. The RFED is in the ON state only during a read operationwhen either a 0 or 1 state are being destroyed during the readingprocess, after which the RFED is programmed back into the 0 or 1 state.Thus for bit storage the RFED is never in the ON state nor in the OFFstate.

In view of the foregoing, it is clearly seen that by choice ofdielectric/electrolyte and electrode materials, the operating window ofa RFED in accordance with the invention can provide a reliable storageor logic device that can be formed at a size much smaller than a singletransistor at any given minimum feature size regime and which can beformed in an extremely compact cross-bar matrix array. If thedielectric/electrolyte layers are kept to a small thickness of under 100(e.g. 16) nanometers, the writing and access times is on the order of afew nanoseconds. Further, by limiting current through the device when ina conductive ON state, the set and reset voltages can be decoupled,allowing an even broader choice of materials while increasing thereliability of the device and allowing non-conductive states to bediscriminated in a simple manner.

In accordance with a perfecting feature of the invention, in an RFED, anindividual switch can be replaced by a modified switch such as thatdepicted in FIG. 23. That is, the modified switch of FIG. 23 could beused for either or both atomic switches in an RFED. The impact of such asubstitution will now be discussed.

Referring now to FIG. 23, a perfecting feature of the invention which isnot necessary to the practice of the invention in accordance with itsbasic principles but which can provide additional functionality andbreadth of application of the invention will now be discussed.Specifically, it has been found by the inventors that if a additionalthin (e.g. 4-7 nm) layer 110 of δ-copper (so-called because the layer isextremely thin even in comparison with other layer (e.g. thedielectric/electrolyte) is applied to the inert electrode of an atomicswitch, both volatile and non-volatile switching behaviors can beselectively produced and the device can be controllably transitionedbetween volatile and non-volatile behaviors by control of the compliancecurrent at which it is operated. If the device is operated at acompliance current below a critical value, the device will exhibitvolatile behavior but if operated at a compliance current above thecritical value, the device will exhibit non-volatile behavior. When anon-volatile copper nanofilament is reset as described above, the devicewill exhibit volatile behavior if and when it is thereafter operated ata compliance current below the critical value. These unexpectedproperties in an atomic switch permit numerous new applications in logiccircuits, memristive circuits such as the memory array discussed indetail above, and chaotic circuits and neural networks as will bediscussed below. Importantly, the atomic switch in accordance with thisperfecting feature of the invention can perform as either an NVM, SRAMor a DRAM and SRAM and DRAM cells can be provided in the same device andmay be collectively referred to as a resistance RAM (ReRAM or RRAM).Moreover, such a memory device can be dynamically configured to havedifferent relative SRAM and DRAM capacities. Additionally, thecontrollable volatility switching device can be fabricated in a verysmall area such as 5 nm×5 nm and thus provides a substantial potentialintegration density advantage over other transistor-based storage cellssuch as the six-transistor SRAM cell alluded to above which are much toolarge for high integration density applications.

FIG. 23 also includes an I-V plot of the controllable volatility deviceoperated in a volatile storage mode. As with the atomic switchesdescribed above, the conductive bridge is formed at V_(set) with acompliance current limit applied. However, as the voltage is swept backto zero, the copper nanofilament dissolves and a high resistance stateis assumed spontaneously. Therefore, no reset operation is required tochange the storage state of the device. This process is highlyrepeatable as indicated by the I-V. plots of FIG. 24 which show the samedata but differ in scale. In other words, the atomic switch in thevolatile switching mode behaves like a semiconductor diode. Thepractical impact is that with one additional mask during the processingof a single switch, predetermined cells in a cross-bar matrix can berendered volatile while other cells exhibit non-volatile behavior. Thus,the cross-bar matrix array can provide both memory cells and diode-likedevices.

While not wishing to be held to any particular theory of how volatileswitching is produced in such a cell, it is postulated that thespontaneous dissolution of copper filaments (CF) is an imbalance betweenthe electric field-supported Cu⁺ copper ion flux and the self-diffusionof copper in the CF and through the interface between the conductivebridge and the δ-Cu layer 110 on the platinum electrode, as conceptuallyshown in FIG. 25. In the absence of a δ-Cu layer 110, as in the atomicswitches described above, there is no copper diffusion flux from thecopper conductive bridge at the interface with the platinum inertelectrode. The δ-Cu layer appears to enable such a diffusion flux fromthe copper conductive bridge into the δ-Cu layer 110 and the flux issupported by the enhanced local temperature resulting from Joule heatingand tends to remove copper from the CF. If the rate of removal of copperfrom the CF is larger than the rate of field-supported copper ion flux,the CF dissolves spontaneously. The diffusion flux into the δ-Cu layerappears to increase with the thickness of the δ-Cu layer and the abovehypothesis appears to be verified by forming a Cu/TaO_(x)/Cu device inwhich CF formation could not be observed.

By the same token the thickness of the δ-Cu layer appears to berelatively critical to the production of controllable volatility. If theδ-Cu layer is less than about 4 nm, only non-volatile switching behaviorcan be achieved. In such a case, the δ-Cu layer is too thin to supportsubstantial diffusion and depletion of copper from the CF. The behaviorof such a device is substantially the same as a device in which the δ-Culayer is omitted. If the thickness of the δCu layer is in the range of4-8 nm, controllably volatile switching behavior is obtained. Above 8 nmδ-Cu layer thickness, no CF formation is observed as the limiting caseof Cu/TaO_(x)/Cu geometry is quickly approached.

The Cu flux in the δ-Cu layer is mainly controlled by the thickness ofthat layer, but is also a function of the current passing through the CFby virtue of increased temperature due to the Ohmic or Joule heating dueto the current which increases diffusivity. Thus, by control ofthickness of the δ-Cu layer and control of compliance current the timeover which spontaneous reset occurs can be controlled to some degree.

The copper ion flux is mainly controlled by the applied voltage butalso, to a degree, by the supply of Cu atoms which is maximized by thebulk copper electrode. However, it is possible to control the flux ofcopper ions by replacing the bulk copper electrode with an inertelectrode having a second layer of δ-Cu in contact with thedielectric/electrolyte as shown in FIG. 26.

Referring now to FIG. 27, another potentially useful property of thecontrollable volatility device will be discussed. Specifically, thedevice can be operated in two different I-V regimes having differentresistance characteristics. With the positive voltage sweep beginning a0V, as shown in FIG. 27, when the bias voltage reached V_(set) the CFwill be formed and will have a resistance of about 1000Ω. As soon as thebridge is formed substantial current in excess of 200 mA will flowthrough the bridge if not limited to a compliance current level; leadingto Joule heating and increased diffusivity in the δ-Cu layer copperremoval from the bridge and resistance increase. At any given voltage,the power is given by P=V²/R, and the bridge will tend to stabilize tobalance the copper and copper ion fluxes, indicated by the slope ofR_(on1). During the voltage sweep back to zero, the bridge issubstantially stable but of sharply increased resistance as the bridgedissolves, indicated by the slope R_(on2).

Referring now to FIG. 28 the transition between volatile andnon-volatile witching behavior will now be discussed. When thecontrollable volatility device is operated at currents above about 1 mAa transition from volatile to non-volatile behavior is observed. Twoset/reset cycles of operation of the controllable volatility device areshown in FIG. 28 in which the current is limited to a compliance currentlevel of slightly more than 1 mA. It is seen that a voltage excursion toabout −0.5 V to −0.6 V and a current of about −2.0 mA is required toachieve reset. However, once reset, the device can again be made toexhibit volatile behavior if the currents are limited to a compliancecurrent of below several hundred μA.

Some additionally improved properties of the asymmetrical controllablevolatility device. Specifically, as shown in FIG. 29, the cumulativestatistical distribution of set voltage at which proper CF formationoccurs over a range of V_(set) voltages is graphically illustrated forthe controllable volatility device of FIG. 23 and an atomic switchillustrated in FIG. 1. It is seen that the range of V_(set) voltages atwhich setting may occur is much smaller for the controllable volatilitydevice than for the atomic switch of FIG. 1.

Similarly, the R_(on) and R_(off) resistance distributions are muchtighter for the controllable volatility device that for the atomicswitch FIG. 1 without the δ-Cu layer. Tight distributions are necessaryfor commercial circuit applications. As alluded to above, by using δ-Culayer thicknesses on both the copper an inert electrodes to control therespective copper and copper ion fluxes, the switching properties of thecontrollable volatility switching device can be optimized for a widerange of applications. A variant structure of substantially universalapplicability where controllable volatility or particularly tightswitching characteristics are advantageous is schematically shown inFIG. 31.

Further fine-tuning of V_(set) can be achieved by the choice ofelectrode materials since the energy required for converting migratingions to neutral atoms (e.g. copper and/or oxygen) are observed to bedifferent for some different metals. For example, the overvoltagepotential of an electrochemical reaction to convert O⁻² to O₂ forplatinum is 0.77 V; 0.56 V for nickel, 1.02 V for gold and 0.93 V forpalladium. Thus the switching device in accordance with this perfectingfeature of the invention provides much improved performance compared toother atomic switch designs but can be optimized to closely matchrequirements for a wide range of applications.

The ability to adjust the electrical responses of RFEDs or controllablevolatility devices is also useful in developing various logic functions.While a RFED or controllable volatility switching device is, in essencea transmission gate, simple logic functions can be achieved by responsesto sequential inputs, arrays of cells that can be pre-set to achievelogical combinations of concurrent inputs, responses to differentvoltage levels or time durations and the like. The variety of theparameters of electrical signals to which RFEDs and/or controllablevolatility device can respond also provides qualities that areparticularly useful for neural networks as will now be discussed asillustrative of the variety of ways in which the invention can beemployed to perform logic functions. In this application, singleswitches are joined serially such that the floating electrode can beconfigured as a bi-layered composite of an inert and an activeelectrode.

It is important to keep in mind that a major characteristic of thememristive switch (r-switch) is that the state variable is not a voltagebut the total quantity of charge injected into the soliddielectric/electrolyte sandwiched between an active electrode and aninert electrode. Therefore the output/response characteristic of anr-switch depends on the prehistory of its operation. For example, aslight variation of the I-V characteristic is observable over twosuccessive set-reset cycles of the controllable volatility switch inFIGS. 27 and 28. More generally, at a given applied voltage, V_(ap),across the switch, a finite time, τ_(crit), is required to establish aCF which varies as a function of both voltage and dielectric/electrolytethickness (generally 8-32 nm). When a CF is established in a given cell,the cell switches abruptly from a high resistive state, R_(off), (forexample, between 1 MΩ (e.g. for oxygen deficient TaO_(x)) to 300 GΩ(e.g. for Ta₇O₅₎) to a low resistive state (typically 80Ω to 400Ω). Thisyields an on/off resistance ratio of about 10⁶ to 10⁹, far higher thancan be obtained from transistors particularly when fabricated at smallsizes.

For a sufficiently large V_(ap)>3V₀, there is an exponentialrelationship between V_(ap) and τ_(crit):τ_(crit)=τ₀exp(−V _(ap) /V ₀)  (1)where τ₀ and V₀ are constants that depend upon cell geometry andmaterials, respectively. For 32 nm thick tantalum oxide baseddielectric/electrolytes described above and copper as the activeelectrode, τ₀=120 seconds and V₀=300 mV have been extracted fromexperimental data. For example, at V_(ap)=1.0 V, τ_(crit)=1.24 seconds.Thus, when several switches are connected in series to form a voltagedivider and the on and off resistances selected appropriately, it ispossible to determine, for a given applied voltage across theseries-connected switches, the time that each switch will transitionfrom the off state to the on state and the transitions will occur in acascaded manner at predetermined times and produce predetermined currentlevels. The reverse process, referred to as unlearning, can also be madeto progress in a cascaded manner through choice of materials and cellgeometry to produce different reset current for the cells. The cell withthe lowest reset current will rupture first; increasing the resistancein the serial string and reducing current until current or reset voltageis increased. Such electrical behavior is particularly useful forneural, adaptive and artificial intelligence circuits where a responseis learned from prior stimuli.

Referring now to FIG. 32, an exemplary series connection of three atomicswitches is schematically shown together with the chosen electrolyte andgeometry of each cell. In this application, single switches are joinedserially (e.g. the floating electrode is now a bi-layered composite ofan inert and an active electrode). It should be appreciated that aserial connection of any number of atomic switches can be fabricatedusing the method described above in connection with FIG. 8 but addingadditional active electrode layers on each inert electrode layer (toprovide serial rather than anti-serial connections) and additionalpatterning of the interior electrode layers. Moreover, the serialconnection of r-switches can be fabricated at a single site of across-bar matrix although the area required will be the area of thematrix site must accommodate the largest area switch in the seriesconnection. However, with proper choice of materials all three switchescould be of minimum feature size. The area of the respective switches isprincipally important to the respective R_(off) values which decreasewith increased switch area. As switches sequentially become conductivethe reduction of resistance will determine the increase in current flowin the series connection for the voltage applied (which must initiallybe large; approaching the sum of the set voltages of the cells of theseries connection) and the area is thus important to the time periodprior to the setting of the next switch in the sequence. Different areasare employed in the series connection of r-switches shown in FIG. 32 toallow use of tantalum oxide-based dielectric/electrolyte in all of ther-switches.

The electrical response of the series connection of r-switches of FIG.32 is shown for two different applied voltages of 1.2 V and 2.0 V inFIGS. 33A and 33B, respectively. It can be seen that the cascadedresponse is quite similar but that the cascaded sequence of switchingoccurs far more rapidly at the higher voltage in accordance with theexponential response indicated by Equation (1), above. In this regard,it should be noted that the cascaded switching accelerates as switchingof the individual switches occurs; thus increasing the voltage appliedto the switches remaining in the off state. The current levels producedare shown analytically in FIG. 33C. It should be noted that the changesin current level, while not instantaneous, are very rapid and are quitestable at any level; allowing the state of the circuit to beunambiguously read although the point (II) at which r-switch 2 becomeconductive is small due to the particular choice ofdielectric/electrolyte and occurs almost simultaneously with r-switch 3becoming conductive at higher voltage.

Recalling that the state variable is the amount of charge injected intothe solid electrolyte and the amount of charge is largely a function ofV₀, this behavior suggests that accurate control of the state ofswitches can be achieved using pulses and that the order of r-switchesbecome conductive could be controlled by pulses of different heights andwidths as shown in FIG. 34.

Learning and unlearning operations can be made specific to eachrespective r-switch and can also be accelerated by the addition of twoanti-serial branch connections to the serial connection of FIG. 32. Sucha circuit is schematically shown in FIG. 35A and fabrication incross-bar matrix form is shown in cross-section is FIG. 35B. It shouldbe noted that this circuit requires only three adjacent cross-bar matrixsites and can also be formed at minimum feature size by a processsimilar to that described above in connection with FIG. 8 but differingin the patterning of interior inert electrode layers to form RFEDs.

In view of the foregoing, it is seen that the invention provided analternative to transistor-based storage and logic cells that can beformed at much smaller areas and integrated much more densely.Controlled volatility allows both static and dynamic memory cells to beprovided in identical structures in the same device on the same array.Arbitrary logic functions can be derived in a variety of ways andneural, adaptive and artificial intelligence circuits capable oflearning or unlearning can be easily provided.

While the invention has been described in terms of a single preferredembodiment and perfecting features, those skilled in the art willrecognize that the invention can be practiced with modification withinthe spirit and scope of the appended claims.

The invention claimed is:
 1. A method of operating an atomic switchcomprising steps of applying a threshold voltage across said atomicswitch to form a conductive filament that renders said atomic switchconductive, limiting current through said atomic switch to a level lessthan a current level required to rupture said filament and render saidatomic switch substantially non-conductive and of relatively higherresistance than in said conductive state to maintain said atomic switchin said conductive state, and terminating said step of limiting currentwhile said voltage applied to said atomic switch is equal to or greaterthan a threshold voltage required to render said atomic switchsubstantially non-conductive and of said relatively high resistance dueto rupturing of said filament.
 2. The method as recited in claim 1,wherein said atomic switch is one of two atomic switches connected inanti-serial relationship.
 3. The method as recited in claim 2, includinga further step of applying a reset voltage having a magnitude greaterthan a reset threshold to said two atomic switches while performing saidstep of limiting current whereby an ON state of said two of atomicswitches is maintained.
 4. The method as recited in claim 3, including afurther step of terminating said step of limiting current while avoltage having a magnitude greater than said reset threshold is applied,whereby a reset voltage of one atomic switch is decoupled from a setvoltage of the other atomic switch.
 5. The method as recited in claim 1,wherein said atomic switch is one of two or more atomic switchesconnected in serial relationship.
 6. The method as recited in claim 5,including a further step of applying a reset voltage having a magnitudegreater than a reset threshold while said atomic switch is in an ONstate and performing said step of limiting current whereby an ON stateof said atomic switch is maintained.
 7. The method as recited in claim6, including a further step of terminating said step of limiting currentwhile a voltage having a magnitude greater than said reset threshold ofsaid atomic switch is applied, whereby said reset voltage of said atomicswitch is decoupled from a set voltage of another atomic switch.
 8. Anatomic switch comprising an inert electrode, an active or further inertelectrode spaced from said inert electrode, a soliddielectric/electrolyte filling a space between said inert electrode andsaid active or further inert electrode, and means for limiting currentthrough said atomic switch when said atomic switch is in a conductivestate to a level less than a current level that renders said atomicswitch non-conductive and terminating said limiting of current.
 9. Theatomic switch as recited in claim 8, further including a δ-copper layeron said inert electrode.
 10. The atomic switch as recited in claim 9,wherein said δ-copper layer has a thickness of less than 4 nm.
 11. Theatomic switch as recited in claim 9, wherein said δ-copper layer has athickness of between 4 nm and 8 nm.
 12. The atomic switch as recited inclaim 9, wherein a compliance current level for limitation of current bysaid means for limiting current is chosen such that the atomic switch isselectively operated in a volatile or non-volatile mode.
 13. The atomicswitch as recited is claim 8, connected in anti-serial relationship withanother atomic switch to form a resistive floating electrode devicehaving a common inert electrode.
 14. The atomic switch as recited inclaim 8, wherein said atomic switch is formed within a cross-bar matrixof atomic switches.
 15. The atomic switch as recited in claim 10,wherein said atomic switch is formed within a cross-bar matrix of atomicswitches.
 16. A logic device comprising three or more serially connectedatomic switches wherein said serially connected atomic switches arestacked with an inert electrode of one atomic switch being directlyadjacent to an active metal electrode of another atomic switch, saidthree or more atomic switches exhibiting OFF state resistances thatdiffer from each other, ON state resistances that differ from each otherand reset currents that differ from each other, and an additional atomicswitch connected to a node between two of said three or more seriallyconnected atomic switches.
 17. The logic device as recited in claim 16,wherein voltages are applied to said additional atomic switch as pulsesof different selected amplitude and/or duration.
 18. The logic device asrecited in claim 16, wherein voltages are applied to said seriallyconnected atomic switches as pulses of different selected amplitudeand/or duration.
 19. A method of operating an anti-serial connection oftwo atomic switches, said method comprising steps of applying athreshold voltages across said atomic switches to render said atomicswitches conductive, limiting current through said atomic switches to alevel less than a current required to render either of said atomicswitches non-conductive regardless, within limits, of voltage appliedacross said anti-serial connection of said two atomic switches, andterminating said step of limiting current while an applied voltageapplied to either of said atomic switches is of a magnitude greater thana threshold voltage required to render a respective one of said twoatomic switches non-conductive.